1. Field of the Invention
The present invention generally relates to a fitting or a fitting structure of a synthetic resinous material mainly used in a tubing system in a high technology field or industry such as fabrication of semiconductors, for example, for transferring a high-temperature, high-pressurized special chemical fluid which should be under strict control because of its environmental impact, for example.
2. Prior Art
There are known fitting structures for plastic pipes or tubes, which use fluid-tight screws, ferrules, or a variety of other fluid-tight structures. Because of inherent natures of fluoroplastics, fitting structures made of a fluoroplastic material, for example, TFE (tetrafluoroethylene), PFA (copolymer of polytetrafluoroethylene and perfluoroalkylvinylether), or FEP (copolymer of polytetrafluoropropylene), are often used for transferring high-temperature, high-pressurized chemical fluids, particularly chemical liquids which need careful handling. However, conventional fluoroplastic fitting structures involve the problem of leakage of fluids. Namely, these fitting structures repeatedly experience expansion and contraction during the heat cycle, and creeps are produced in the fitting structures. These creeps result in making microscopic gaps between connected elements, and this is a major reason of the leakage problem. It is extremely difficult to prevent these gaps from being made.
To solve the problem a tube fitting as shown in FIG. 7 has been proposed. The prior art fitting shown in FIG. 7 is used for connecting a tube to an associated device, for example. In FIG. 7, the reference numeral 1 represents a tube made of a fluoroplastic material. One end or a forward end section of the tube 1 is folded back to outwardly overlap the remainder main portion of the tube 1 to form a folded-back portion of a predetermined length. As a result, the tube 1, in the folded shape, defines a folded-back end which is U-shaped in its cross section.
A fitting for connecting the tube 1 is made up of an insert 10, a fitting body 6 and a ring member 8 which are all made of a fluoroplastic material.
The insert 10 has a tubular shape and it is mounted on the tube 1 such that one end section (lower end section in FIG. 7) of the insert 10 sits between the main body of the tube 1 and the foldedback portion thereof.
The fitting body 6 includes an annular groove 6a whose bottom is U-shaped in its cross section to fit the U-shaped forward end of the tube 1; a cylindrical portion 6c encircling the annular groove 6a to define an inner wall surface 6b for contact with the outer wall of the folded-back portion of the tube 1 and an outer wall surface threaded to form a threaded portion 6f; and a connecting portion 6e threaded on its outer circumferential surface to form a threaded portion 6d for connecting the fitting body 6 to an associated device or apparatus.
The ring member 8 includes an inner ring portion 8a for engagement with the other end (upper end in FIG. 7) of the insert 10, and an outer cylindrical portion 8b whose inner wall surface is threaded to form a threaded portion 8c for threading engagement with the threaded portion 6f of the fitting body 6.
For fabricating the fitting structure, one end section of the tube 1 is first folded back outwardly by a predetermined length, and the insert 10 is inserted between the main portion and the folded-back portion of the tube 1. Then, the semi-assembly of the tube 1 and the insert 10 is joined to the fitting body 6. After that, the portion 6f of the fitting body 6 is threadedly engaged with the portion 8c of the ring member 8 until the insert 10 tightly compresses the U-shaped forward end of the tube 1 onto the annular groove 6a of the fitting body 6.
As a result, the tube 1 is firmly connected to the fitting, and the above-mentioned leakage problem is removed.
However, the prior art fitting structure is bulky in diameter because of the radially overlapped arrangement of the insert 10, the cylindrical portion 6c of the fitting body 6 and the outer cylindrical portion 8b of the ring member 8 around the tube 1 with threads being formed on the portions 6c and 8b for firm engagement between the fitting body 6 and the ring member 8. With the prior art fitting, the diametrical bulkiness of the fitting will make it impossible to arrange a plurality of tubes closely side by side for connection with the associated device.